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Simon J. Thorpe, Sèbastien M. Crouzet, Marc J. M. Macé, Nadège Bacon-Macé, Michèle Fabre-Thorpe; Masking in a high-level gender discrimination task is essentially entirely pre-cortical. Journal of Vision 2009;9(8):546. doi: https://doi.org/10.1167/9.8.546.
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Our ability to detect animals in briefly flashed natural scenes is reduced when a high contrast dynamic mask is presented within about 40 ms and masking is complete at the shortest delays (Bacon-Macé et al.,Vision Research, 2005). It is generally assumed that such masking involves high-level interference. To investigate more precisely when and where this interference occurs, we designed a task that systematically varied mask-target delay and the type of presentation (monocular, binocular and dichoptic). A large number (216) of face photographs were briefly flashed (1 frame with a 200 Hz vertical refresh rate, i.e. for 5ms), and subjects reported whether the face was male or female. Masking was produced using 4 frames (20 ms) of the high contrast spatial patterns used in the Bacon-Macéeacute; et al study. When the target face and masks were presented binocularly, or monocularly through the same eye, we obtained a classic masking function with accuracy dropping to 50% chance level when the masks occurred immediately before or after the target. Forward masking was still very strong with 4 blank frames between the mask and target (i.e. ISI = 20 ms), dropped by about one third at 40 ms, and only disappeared when the interval was 100 ms. Backwards masking was also complete with the shortest intervals, but did not last as long, with performance reaching nearly normal levels with a gap of 40 ms. In contrast, when the target and masks were presented to different eyes (dichoptic presentation), there was essentially no masking effect whatsoever, even when the two stimuli are presented simultaneously. The inevitable conclusion is that the masking is produced at levels in the visual system where inputs from the two eyes have yet to be combined, i.e. in the retina, geniculate and layer IV of primary visual cortex.
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